Method for membrane deposition

ABSTRACT

A method and apparatus for applying a uniform membrane coating to a substrate, such as a honeycomb structure, having a plurality of through-channels, wherein the through-channels have an average diameter of less than or equal to 3 mm. The method includes providing a liquid precursor comprising membrane-forming materials to the substrate and applying a pressure differential across the substrate. The pressure differential causes the liquid precursor to travel uniformly through the through-channels, depositing the membrane-forming materials on the walls of the through-channels and forming the membrane on the walls of the through-channels. The apparatus includes a chamber capable of holding the substrate and of maintaining a pressure differential across the plurality of through-channels.

BACKGROUND OF INVENTION

The invention relates to the deposition of coatings in channels. Moreparticularly, the invention relates to the deposition of membranecoatings in honeycomb channels having small diameters. Even moreparticularly, the invention relates to a method and apparatus fordepositing membrane coatings in honeycomb channels having smalldiameters.

A variety of coating methods, including dip-coating, slip casting, andspin coating have been used to deposit layers of materials—particularlymembrane materials—on porous substrates or support structures. Suchmethods work well for coating flat surfaces, external surfaces oftubular structures, or internal surfaces of large diameter tubes.

However, the application of membrane coatings to multiple small sized(≦3 mm diameter) channels in a monolithic substrate or support presentschallenges that are difficult to overcome using the previously mentionedmethods. Such challenges include the introduction of viscous coatingsolutions into small channels, providing uniform coatings along thelength and around the circumference of the channels, and controllingdeposition conditions, such as residence time. In addition, the abovemethods are not effective in coating such channels using reactiveprocesses, such as electroless plating, hydrothermal synthesis, and thelike.

Whereas wash coating has been used to deposit catalyst layers onhoneycomb substrates having small channels and high densities, thisprocess is not applicable to the membrane coating process for severalreasons. First, wash coating typically results in a coating layer havinga number of cracks and a thickness that is greater than that desired formembrane films. Secondly, the length of the honeycomb structure that canbe coated by wash coating is limited to about 6 inches (about 15 cm).

Presently available coating techniques are unable to provide uniformmembrane coatings for small channels of honeycomb structures of longerlength. Therefore, what is needed is a method of applying a uniformmembrane coating to the channels of such honeycomb structures. What isalso needed is an apparatus that enables the application of suchmembrane coatings to honeycomb structures having small channels.

SUMMARY OF INVENTION

The present invention meets these and other needs by providing a methodand apparatus for applying a uniform membrane coating to a substrate,such as a honeycomb structure, having a plurality of through-channels.The method includes providing a liquid precursor comprisingmembrane-forming materials to the substrate and applying a pressuredifferential across the substrate. The pressure differential causes theliquid precursor to travel uniformly through the through-channels,depositing the membrane-forming materials on the walls of thethrough-channels and forming the membrane on the walls of thethrough-channels. The apparatus includes an inlet that uniformlydistributes a liquid precursor to the honeycomb structure of thesubstrate, a chamber capable of holding the substrate and maintaining apressure differential across the plurality of through-channels, and anoutlet.

Accordingly, one aspect of the invention is to provide an apparatus fordepositing a membrane. The apparatus comprises: a liquid precursorsource adapted to contain a liquid precursor; a chamber; and apressurization system coupled to the chamber. The chamber comprises: amidsection adapted to support a substrate having a first end, a secondend, and a plurality of through-channels extending through the substratefrom the first end to the second end; an inlet section adjacent to themidsection in contact with the first end of the substrate, wherein theinlet is in fluid communication with the liquid precursor source and thefirst end of the substrate, and wherein the inlet section is capable ofproviding a uniform distribution of the liquid precursor to the firstend of the substrate; and an outlet section adjacent to and in fluidcommunication with the second end of the substrate, wherein the outletsection is capable of providing a uniform discharge of a fluid from thesecond end of the substrate resulting in removal of the fluid from thechamber. The pressurization system provides a pressure differentialbetween the first end and the second end of the substrate through theplurality of through-channels.

A second aspect of the invention is to provide a chamber for depositinga membrane. The chamber comprises: a midsection adapted to support asubstrate having a first end, a second end, and a plurality ofthrough-channels extending through the substrate from the first end tothe second end; an inlet section adjacent to the midsection in contactwith the first end of the substrate, wherein the inlet section is influid communication with the first end of the substrate, and wherein theinlet section is capable of providing a uniform distribution of a liquidprecursor to the first end of the substrate; and an outlet sectionadjacent to and in fluid communication with the second end of thesubstrate, wherein the outlet is capable of providing a uniformdischarge of fluid from the second end of the substrate resulting inremoval of the fluid from the chamber; and wherein the chamber iscapable of maintaining a pressure differential between the first end andthe second end through the plurality of through-channels.

A third aspect of the invention is to provide an apparatus fordepositing a membrane, the apparatus comprising: a liquid precursorsource adapted to contain a liquid precursor; a vertically orientedchamber; and a pressurization system coupled to the chamber. The chambercomprises: a midsection adapted to support a substrate having a firstend, a second end, and a plurality of through-channels extending throughthe substrate from the first end to the second end; an inlet sectionlocated at the bottom of the chamber and below the midsection, whereinthe inlet section is in contact with the first end of the substrate andin fluid communication with the liquid precursor source and the firstend of the substrate, and wherein the inlet section is capable ofproviding a uniform distribution of the liquid precursor to the firstend of the substrate; an outlet section located at the top of thechamber and above the midsection, and wherein the outlet section is influid communication with the second end of the substrate, wherein theoutlet section is capable of providing uniform discharge of a fluid fromthe second end of the substrate resulting in removal of the fluid fromthe chamber. The pressurization system provides a pressure differentialbetween the first end and the second end through the plurality ofthrough-channels.

Another aspect of the invention is to provide a method of forming amembrane in a plurality of through-channels disposed in a substrate. Themethod comprises the steps of: providing the substrate to a chamber, thesubstrate having a first end, a second end, and a plurality ofthrough-channels extending through the substrate from the first end tothe second end, wherein the chamber has an inlet section, a midsection,and an outlet section, wherein the substrate is disposed in themidsection such that the first end is adjacent to and in fluidcommunication with the inlet section and the second end is adjacent toand in fluid communication with the outlet section; providing a liquidprecursor to the inlet section, wherein the liquid precursor comprisesmembrane-forming materials; providing a pressure differential betweenthe inlet section and the outlet section, wherein the pressuredifferential causes the liquid precursor to flow uniformly through theplurality of through-channels; and forming the membrane on a surface ofthe plurality of through-channels.

These and other aspects, advantages, and salient features of the presentinvention will become apparent from the following detailed description,the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of cross-sectional view of asubstrate having a plurality of through-channels that are coated with amembrane;

FIG. 2 is a schematic representation of an apparatus for depositing amembrane on the walls of a plurality of through-channels of a substrate;

FIG. 3 is a schematic representation of one embodiment of an apparatusfor depositing a membrane on the walls of a plurality ofthrough-channels of a substrate;

FIG. 4 is a schematic representation of another embodiment of anapparatus for depositing a membrane on the walls of a plurality ofthrough-channels of a substrate;

FIG. 5 shows optical microscopy images (4× magnification) of crosssections of α-alumina membrane coatings deposited on the walls of 0.75mm diameter through-channels in a monolithic support;

FIG. 6 a is a plot of membrane coating thickness as a function of radialand longitudinal location;

FIG. 6 b is a diagram showing the locations of the through-channelscorresponding to the data points in FIG. 6 a;

FIG. 7 a is an photograph of a palladium membrane coating deposited onthe through-channels of a monolithic substrate

FIG. 7 b is a scanning electron microscopy (SEM) image (3,000×magnification) of a cross-section of the palladium membrane shown inFIG. 7 a;

FIG. 8 a is an SEM image (2,500× magnification) of the as-plated surfacetexture of the palladium membrane shown in FIGS. 7 a and 7 b; and

FIG. 8 b is an SEM image (5,000× magnification) of the surface textureof the palladium membrane shown in FIGS. 7 a and 7 b after annealing at450° C. for two days.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise any number of those elements recited, either individually or incombination with each other. Similarly, whenever a group is described asconsisting of at least one of a group of elements and combinationsthereof, it is understood that the group may consist of any number ofthose elements recited, either individually or in combination with eachother.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing a particular embodiment of the invention and are not intendedto limit the invention thereto. Turning to FIG. 1, a schematicrepresentation of a cross-sectional view of a substrate 120 having aplurality of through-channels 122 that are coated with membrane 123.Some of the challenges faced in applying membrane coatings 123 to thewalls of though-channels 122, particularly where each of thethrough-channels 122 has a diameter of up to about 3 mm, of a monolithicsubstrate 120 include non-uniform distribution of the membrane coating123 throughout all of the through-channels 122, non-uniformity of themembrane coating 123 along the length of each through-channel 122,non-uniform distribution of membrane coating 123 over thecross-sectional area of each through-channels 122, difficulties inintroducing viscous coating solutions into the plurality ofthrough-channels 122, and difficulty in controlling depositionconditions such as residence time.

Problems associated with obtaining uniform membrane coatings 123 overthe cross-sectional area of each through-channel 122 are schematicallyshown in FIG. 1. While a predetermined thickness and uniformity ofmembrane coating 122 is desired ((a) in FIG. 1), prior art methods ofcoating through-channels 122 typically yield membrane coatings ofuncontrolled thickness ((b) in FIG. 1) or asymmetry of membrane coating122 ((c) in FIG. 1).

An apparatus 200 for depositing a membrane on a substrate 210 isschematically shown in FIG. 2. Substrate 210 comprises a plurality ofthrough-channels 212 extending from a first end 214 to a second end 216of substrate 210. In one embodiment, each of the through-channels 212has a diameter of less than about 3 mm and, preferably, less than about2 mm. In another embodiment, substrate 210 is a honeycomb structure inwhich individual through-channels 212 have diameters in a range fromabout 0.5 mm up to about 2 mm. The density of through-channels 212 insubstrate 210 is in a range from about 50 channels per square inch(cpsi) up to about 600 cpsi.

Apparatus 200 comprises a chamber 220 adapted to accommodate substrate210 and maintain substrate at a predetermined pressure, a liquidprecursor source 230 in fluid communication with chamber 220, and apressurization system (not shown) coupled to chamber 220.

The pressurization system provides a pressure differential between firstend 214 and second end 216 of substrate 210 through the plurality ofthrough-channels 212. The pressurization system may comprise those meansknown in the art for providing positive or negative pressure in achamber. Such means include, but are not limited to, mechanical pumps,peristaltic pumps, vacuum umps, hydraulic pressurization units, and thelike. In one embodiment, the pressurization system is in fluidcommunication with at least one of inlet section and the outlet section.

In one embodiment, the pressure differential provided by thepressurization system is sufficient to maintain a predetermined linearflow velocity of the fluid precursor through the plurality of monolithicthrough-channels 212. The actual linear flow velocity depends in partupon the viscosity and composition of the liquid precursor material andthe dimensions of the plurality of through-channels 212. The linear flowvelocity is typically in a range from about 0.01 cm/s to about 200 cm/s.In one embodiment, the linear flow velocity is in a range from about 1cm/s up to about 200 cm/s.

In one embodiment, the pressurization system maintains a positivepressure difference between the inlet and the outlet. The positivepressure difference is typically in a range from about 1,000 Pa up toabout 1,000,000 Pa (10 bar). In one embodiment, the positive pressuredifference is in a range from about 1,000 Pa up to about 200,000 Pa (2bar). In a particular embodiment, the pressure differential is about1,700 Pa. It will be appreciated by those skilled in the art that thepressure differential that is actually employed depends at least in partupon through-channel diameter and the viscosity of the liquid precursor.The pressure difference may be provided by maintaining a positivepressure in inlet section 224. In one non-limiting example, thepressurization system creates a pressure differential by pumping theliquid precursor in inlet section 224. Alternatively, the pressuredifference may be provided by reducing pressure in outlet section 226.In this embodiment, the pressurization system may be used to generate avacuum in outlet section 226.

The liquid precursor source 230 provides a liquid precursor to chamber220. The liquid precursor comprises contains materials or nutrients thatare necessary to form the membrane. The liquid precursor may be either asolution or a suspension or slurry of solid materials in a carrierliquid. The carrier liquid may be either water-based or organicsolvent-based. The materials or ingredients of the liquid precursor mayinclude solid particles such as, but not limited to, alumina or otherceramic materials, metals, dispersion agents, anti-cracking additives,organic templates, precursors of the membrane materials, and the like.

Chamber 220 includes a midsection 222, an inlet section 224, and anoutlet section 226. Chamber 220 may be formed from glass, steel, or anyother material that is capable of supporting substrate 210 andmaintaining a pressure differential between first end 214 and second end216 of substrate 210. Midsection 222 is adapted to support substrate 210during deposition of the membrane and provide a fluid seal around theouter perimeter of substrate 210 such that inlet section 224 and outletsection 226 are in substantial fluid communication only through theplurality of through-channels 212 in substrate 210 and the pressuredifferential is present across the plurality of through-channels 212.Midsection 222 has an aspect ratio (ratio of length to diameter) that isgreater than 1.

Inlet section 224 is adjacent to the end of midsection 222 that is incontact with first end 214 of substrate 210. Furthermore, inlet section224 is in fluid communication with the liquid precursor source 230 andfirst end 214 of substrate 210. Inlet section 224 serves as adistribution section in which the liquid precursor is distributeduniformly over the cross-section of first end 214 of substrate 210,thereby enabling the liquid precursor to flow through each of theplurality of monolithic through-channels 212 at nearly identical rates.To accomplish this, inlet section may include at least one of an openspace, a plenum, baffles, a packed bed of particles or beads, or othermeans known in the art for uniformly distributing a fluid in a chamberor space.

Outlet section 226 is adjacent to the end of midsection 222 that is incontact with second end 216 of substrate 210. Outlet section 226 is alsoadjacent to and in fluid communication with second end 216 of substrate210. Outlet section 226 provides uniform discharge of fluid as it exitsfrom the plurality of through-channels 212 at second end 216 ofsubstrate 210. Uniform discharge of fluid from substrate 210 may beprovided by an open space between substrate 210 and discharge outlet227, or by tapering the region between substrate 210 and dischargeoutlet 227. Outlet section 226 also provides for removal of the fluidfrom chamber 220 through discharge outlet 227.

There is no limitation as to how chamber 220 may be oriented. In apreferred embodiment, shown in FIG. 2, chamber 220 is orientedvertically such that inlet section 224 is located at the bottom ofchamber 220 below midsection 222 and outlet section is located at thetop of the chamber 220 and above midsection 222. This particularorientation facilitates even deposition of a coating—and subsequentmembrane formation—on the walls of the plurality of through-channels212.

Apparatus 200 may further comprise a heat source for maintaining chamber220 and substrate 210 at a predetermined temperature. The heat sourcemay include microwave heaters, liquid heat exchangers, or a resistivelyheated heat source, such as, but not limited to, heating tapes ormantels, furnaces, and the like. The heat source is capable of heatingchamber 120 and substrate up to a temperature of about 500° C.

Another embodiment of apparatus 200 is shown in FIG. 3. Apparatus 300 isadapted to deposit a membrane on the walls of the plurality ofthrough-channels 312 in substrate 310 using a physical depositionprocess. Such a process may be used to deposit, for example, a membranecomprising a ceramic, such as alumina or the like. A suction tube 332connects a liquid precursor source 330 to chamber 320. Liquid precursorsource 330 contains a liquid precursor, which, in one embodiment, maycomprise a water-based solution, ceramic particles, a dispersant, and apolymeric anti-cracking agent. The interior of chamber 320 is pulled toa vacuum using a vacuum pump 340. The vacuum is monitored using a vacuumgauge 342, which is in fluid communication with vacuum/exit line 344,which connects vacuum pump 340 to outlet section 326 of chamber 320. Inone embodiment, vacuum pump 340 is a water vacuum pump or aspirator,which creates a vacuum of about 5 mm Hg. Control valve 334 is used tocontrol the flow rate of the liquid precursor into inlet section 324. Aportion of inlet section 324 is packed with a bed 325 of beads tofacilitate uniform distribution of the liquid precursor over thecross-section of first end 314 of substrate 310, thereby enabling theliquid precursor to flow through each of the plurality of monolithicthrough-channels 312 at nearly identical rates. In one embodiment, bed325 has a depth of about 2 inches (about 5 cm) and comprises alumina orglass beads, each having a diameter of about 1 mm. A flat stainlesssteel mesh 323 supports substrate 310 in midsection 322 above bed 325.

Another embodiment of apparatus 200 is shown in FIG. 4. Apparatus 400 isadapted to deposit a membrane on the walls of the plurality ofthrough-channels 412 in substrate 410 using a chemical depositionprocess such as, for example, electroless plating or hydrothermalsynthesis. Such processes may be used, for example, to deposit amembrane comprising a metal, such as palladium or the like, or ceramics,such as zeolite. In this embodiment, substrate 410 serves as midsection422 of apparatus 400. Liquid precursor source 430 includes two separatechambers 435 and 436, which hold reactive membrane-forming materials.The liquid precursors are pumped from chambers 435 and 436 throughliquid feed line 432 into an optional mixing chamber 437 where theliquid precursors intermix. Inlet section 424 includes an open spacethat allows the reactive membrane-forming materials to be uniformlydistributed to all of the plurality of through-channels 422 in substrate420. The liquid precursors flow upward through the plurality ofthrough-channels 422, where the reactive membrane-forming materialscombine to deposit the membrane (or a pre-membrane coating) on the wallsof the plurality of through-channels 422. The now depleted liquidprecursors exit the plurality of through-channels 422 of substrates 420to outlet section 426, and are discharged into a tank 450.

A chamber for depositing a membrane on a substrate having a plurality ofthrough-channels, as previously described herein, is also provided.

The invention also provides a method of forming a membrane in aplurality of through-channels disposed in a substrate. A substrate isfirst provided to a chamber. The substrate, as previously described, hasa first end, a second end, and a plurality of through-channels extendingthrough the substrate from the first end to the second end. Each of thethrough-channels has a diameter of less than about 2 mm. The chamber, aspreviously described, has an inlet section, a midsection, and an outletsection. The substrate is disposed in the midsection of the chamber suchthat the first end is adjacent to and in fluid communication with theinlet section and the second end is adjacent to and in fluidcommunication with the outlet section of the chamber.

In a second step, a liquid precursor comprising membrane-formingmaterials is provided to the inlet section of the chamber. The liquidprecursor may be either a solution or a suspension or slurry of solidmaterials in a liquid.

A pressure differential is then provided between the inlet section andthe outlet section. The pressure differential causes the liquidprecursor to flow uniformly through the plurality of through-channels.In one embodiment, a positive pressure difference between the inletsection and the outlet section is provided. The positive pressuredifference is typically in a range from about 1,000 Pa up to about1,000,000 Pa (10 bar). In one embodiment, the positive pressuredifference is in a range from about 1,000 Pa up to about 200,000 Pa (2bar). In a particular embodiment, the pressure differential is about1,700 Pa. The pressure difference may be provided by maintaining apositive pressure in the inlet section while the outlet section is atambient pressure. Alternatively, the pressure differential may beprovided by reducing pressure in the outlet section. In this embodiment,the pressure differential is provided by generating a vacuum in theoutlet section.

In a fourth step, a membrane is formed on the surfaces of the pluralityof through-channels. Membrane deposition or formation is carried outusing physical methods, chemical methods, or combinations of bothphysical and chemical deposition methods.

In one embodiment, the fourth step of forming the membrane comprisesforming the membrane using physical deposition. Here, the liquidprecursor is a suspension, slip, or slurry that comprises a plurality ofsolid particles and a carrier liquid. The solid particles comprisemembrane-forming materials. The plurality of solid particles aretransported by the carrier liquid to the surface of the walls of atleast a portion of the plurality of through-channels. Themembrane-forming materials are deposited on the surface of the walls,leaving an intact deposition layer after the carrier liquid isdischarged. The substrate may then be removed from the chamber and thedeposition layer subsequently dried and fired to form the membrane. Oneexample of such a physical deposition process is described in UnitedStates Provisional patent application filed Feb. 27, 2007, by Zhen P.Song et al., entitled “Inorganic Membranes and Method of Making,” thecontents of which are incorporated herein by reference in theirentirety.

In another embodiment, the fourth step of forming the membrane includesforming the membrane using chemical deposition. Here, the liquidprecursor comprises membrane-forming materials that react with eachother or with the substrate to form the membrane. The membrane-formingmaterials are transported from the liquid precursor onto the walls ofthe plurality of through-channels, where the membrane-forming materialsundergo chemical reactions on the channel wall to form a membrane layer.The membrane-forming constituents may react with the wall or with eachother. Non-limiting example of such chemical deposition includeelectroless plating and hydrothermal synthesis.

In a third embodiment, the fourth step of forming the membrane comprisesa method that combines physical and chemical deposition methods so as toform the membrane. A portion of the membrane-forming materials are firstdeposited on the support/substrate surface to form a pre-coating. Themembrane is then synthesized by reacting membrane-forming materials inthe liquid precursor with the pre-coating.

The thickness, texture, and uniformity of the deposited membrane filmsmay be controlled by process conditions. It will be apparent to one ofordinary skill in the art that the process conditions that are actuallyemployed in the deposition or synthesis of such membrane films depend onthe nature of the membrane film and the liquid precursor, as well asother variables. For example, the linear velocity of the liquidprecursor through the plurality of through-channels affects thehydrodynamics and mass transport of the liquid precursor onto the wallsof the plurality of through-channels. In one embodiment, the liquidprecursor flows through the plurality of through-channels at apredetermined linear velocity. The predetermined velocity, in oneparticular embodiment, is in a range from about 1 cm/s up to about 10m/s.

As used herein, “residence time” is the time required for a fluidtraveling through the plurality of through-channels to traverse thesubstrate—i.e., the time between entry and exit of the liquid precursorin the substrate. The residence time is defined as the ratio of thelength of the substrate (or the average length of the plurality ofthrough-channels) to the linear velocity. The residence time, in oneembodiment, is in a range from about 1 second up to about 1 hour.

“Duration time” is the time that the surface of the plurality ofthrough-channels is exposed to the liquid precursor. Coating durationtime relies on deposition kinetics and determines the time needed formembrane deposition or synthesis to occur. The coating duration time maybe in a range from about 5 seconds up to a few weeks, and is preferablylonger than the residence time in order to obtain a uniform membrane.

The temperature at which membrane deposition or synthesis occurs in arange from about room temperature (about 20° C.) up to about 500° C.

The following examples illustrate the features and advantages of theinvention and are in no way intended to limit the invention thereto.

Example 1 Physical Deposition of Porous α-Alumina Membrane ontoMonolithic Channels

Unless otherwise specified, monolithic substrate (also referred toherein as “monolithic supports”) made of α-alumina, each having an outerdiameter ranging from 8.7 mm to 9.2 mm and a length of about 150 mm,were used in this example. Each monolithic support had 19through-channels uniformly distributed over the cross-sectional area ofthe monolithic support. The average diameter of the through-channels was0.75 mm.

The monolithic support was mounted into the apparatus shown in FIG. 3,described hereinabove. A water-based coating solution 331 containingα-alumina particles, a dispersant, and a polymeric anti-cracking agentwas used as the liquid precursor. The coating solution 331 was placedinside a beaker 330. The top of beaker 330 was open to air, and thecontents of beaker 330 were exposed to atmospheric pressure.

A suction tube 332 was immersed in coating solution 331 and connected tothe rest of apparatus 300, and water vacuum faucet 340 was used to pulla vacuum of about 5 mm Hg in apparatus 300. Control valve 334 was usedto control the flow of coating solution 331 into chamber 320 such thatcoating solution 331 flowed into monolithic support 320 gradually over afew seconds.

Monolithic support 320 was soaked in coating solution 331 for about 20seconds, during which time monolithic support 320 was fully immersed incoating solution 331. Monolithic support 320 was then removed fromapparatus 300. Excess coating solution 331 in the plurality ofthrough-channels 322 was removed by spinning monolithic support 320 at525 rpm, leaving behind a coating on the walls of through-channels 322.Monolithic support 320 was then dried at 120° C. The dried monolithicsupport 320 was heated up to 1200° C. at a rate of 1° C./mim, heated at120° C. for 30 minutes, and fired at about 1225° C. for five minutes toform the membrane coating in the plurality of through-channels 322.

The cross-sections of the deposited α-alumina membranes werecharacterized according to the radial position of the through-channelsin the monolithic support and longitudinal position (i.e., along thelength of the through-channels). Longitudinal samples labeled as beingobtained at the “top,” “middle,” and “bottom” of a through-channel werecollected within 1 cm of the top end of the through-channel, at thegeometric middle of the through-channel, and within 1 cm of the bottomend of the through-channel, respectively. FIG. 5 shows opticalmicroscopy images of cross-sections of the α-alumina membrane coatings523 obtained along the length of monolithic support 520. Image (a) inFIG. 5 shows a cross-section of membrane coating 523 deposited on themiddle section (longitudinally) of inner through-channel 531. Image (b)shows a cross-section of membrane coating 523 deposited on the middlesection of outer through-channel 533. Images (c) and (d) show across-sections of membrane 523 deposited on the top section and bottomsections of center through-channel 532, respectively.

The thicknesses of the membrane coatings 523 shown in FIG. 5 are plottedas a function of radial and longitudinal locations in FIG. 6 a. Thelocations of the through-channels corresponding to the data points inFIG. 6 a are shown in FIG. 6 b. The thicknesses of membrane coating 523deposited using the method and apparatus described herein are generallyuniform in both the radial and longitudinal directions. For example, thethickness of the membrane ranges from about 19 μm (outer through-channel610) to about 14 μm (inner through-channel 640) at the top of thethrough-channels; from about 14 μm (outer through-channel 610) to about16 μm (outer through-channel 650) at the middle of the through-channels;and from about 16 μm (inner through-channel 640) to about 25 μm (outerthrough-channel 650). The membrane deposited in outer through-channel615 has the greatest variation of thickness, ranging from 25 μm at thebottom of the through-channel to about 14 μm at the top of thethrough-channel.

Example 2 Microfiltration of Deposited α-Alumina Membrane

Two sets of membranes formed in the manner described in Example 1 wereused to test the filtration of skim milk/water mixtures. One set ofmembranes was formed using a coating solution comprising 4 vol %α-alumina (Sample 2A), and a second set of membranes was formed using acoating solution comprising 6 vol % α-alumina (Sample 2B). Themicrofiltration performance of the α-alumina membranes is shown in Table1.

Less than 0.1% of the protein particles in the skim milk/water mixturehad a size of less than 0.068 μm, whereas 99.9% of the protein particleshad a size of less than 0.409 μm, as measured by a Nanotrac™ lightscattering size analyzer.

The filtration function of a membrane is gauged by the turbidity number(NTU) of the membrane. NTU is a measure of the amount of particulatepasses through a membrane. A NTU value of less than 12 is consideredacceptable for commercially available 200 nm membranes.

Both sets of membrane coatings exhibited similar microfiltrationperformance. The filtration function of the membranes prepared in thisexample is shown by the rejection of 98-99% of the protein particles,with only two of seven samples having NTU values slightly greater than12. The results indicate that the membranes prepared in this exampleherein performed as expected and at a level comparable to commerciallyavailable membranes.

TABLE 1 Microfiltration performance of α-alumina membranes with skimmilk/water mixture. Sample 2A 2 × 4 vol % coating Operating pressure(psi) 24 24 24 32 Sampling time (min) 13.0 19.0 26.0 34.0 Permeationrate (ml/min) 4.3 3.8 3.4 4.4 Permeance (L/m²/h/bar) 88.7 78.0 71.3 68.6Reduction of turbidity (%) 98.5 98.3 98.2 99.0 NTU (turbidity no.) ofpermeate 10.1 11.8 12.2 6.5 Sample 2B 2 × 6 vol % coating Operatingpressure (psi) 24.0 24.0 24.0 Sampling time (min) 8.0 16.0 24.0Permeation rate (ml/min) 3.2 2.9 2.8 Permeance (L/m²/h/bar) 68.9 63.160.5 Reduction of turbidity (%) 98.3 99.1 99.5 NTU (turbidity no.) ofpermeate 12.5 6.8 4.0

Example 3a Electroless Deposition of Pd Thin Film Membrane Using Methodand Apparatus Described Herein

This example illustrates the chemical deposition of a membrane filmusing the method and apparatus described herein.

Palladium and palladium alloys with copper or silver are well known asmembrane materials for hydrogen separation. These membranes have bothhigh flux and high selectivity to hydrogen permeation at temperaturesabove 250° C.

An electroless plating technique was used in the instant method todeposit palladium membrane coatings on monolithic supports. The liquidprecursor or coating solution comprised aqueous-based precursors. Forthe electroless plating of palladium, the aqueous-based precursorsincluded: PdCl₂ (0.04 M); Na₂EDTA.2H₂O) (0.2 M), NH₃.H₂O (25 wt. %) (600ml/L), and N₂H₄.H₂O (80 wt. %) (1.2 ml/L). The palladium membrane wassynthesized by the following chemical reaction on the support surface:

2Pd(NH₃)₄ ²⁺+N₂H₄+4OH⁻→2Pd↓+N₂↑+8NH₃+4H₂O

Deposition of palladium was carried out on monolithic supportscomprising γ-alumina. While the monolithic supports used in theseexamples comprised γ-alumina, it is understood that the electrolessdeposition process could be used to deposit palladium on othermonolithic supports that have at least one γ-alumina coating layer. Anα-alumina support could, for example, be coated with an α-aluminamembrane that is in turn coated with a γ-alumina film for the depositionof a Pd membrane.

Each monolithic support had an outer diameter of 9.5 mm and a length of50 mm. Each monolithic support had 19 through-channels that wereuniformly distributed over the cross-sectional area of the support. Thethrough-channels had an average diameter of 1.0 mm.

The electroless plating process was carried out using the methoddescribed herein and the apparatus shown in FIG. 4. The monolithicsupport 410, 50 mm in length, was placed vertically in chamber 420.Plating solutions 433, 436 were pumped from liquid precursor source 430through feed line 432 into inlet section 424, and flowed upward throughmonolith 410. To allow plating solutions 433, 436 to be uniformlydistributed to all through-channels 412, some open space was left ininlet section 424 between the inlet from liquid precursor source 430 andfirst end 414 of monolith 410. Plating solutions 433, 436 were thendischarged from chamber 420 into tank 450 through line 440. The flowrate was controlled, and was 60 cc/h. The residence time of the platingsolutions in chamber 420 was about two minutes. The overall durationtime for plating was controlled, and limited to one hour.

Example 3b Electroless Plating Deposition of Pd Thin Film Membrane UsingConventional Method (Comparative Example)

For comparison, electroless plating was carried out using theconventional technique. Here a second γ-alumina monolithic support wasimmersed in a beaker containing a plating solution having the samecomposition as plating solutions 433, 436.

Electroless plating using the conventional technique resulted in platingonly the exterior walls of the monolithic support with palladium.

In contrast to the conventional technique, the flow coating processusing the method and apparatus of the present invention resulted in theuniform coating of all through-channels with palladium. Optical and SEMimages of the palladium membrane coating 714 deposited using the instantmethod on the through-channels 712 of monolithic support 710 are shownin FIGS. 7 a and 7 b, respectively. In FIG. 7 a, a portion of monolithicsupport 710 is cut away to reveal the Pd membrane coating 714 depositedalong the length of through-channels 712. The thickness of Pd membranecoating 714 is about 2 to 3 μm.

The monolithic substrate having the Pd membrane coating was subsequentlyannealed for two days at 450° C. SEM images of the palladium membranesurface texture as-plated and after annealing are shown in FIGS. 8 a and8 b, respectively. As seen in FIG. 8 b, the palladium membrane formed adense film as a result of annealing. A thin dense membrane such as thatshown in FIG. 8 b is known to provide both high flux and selectivity,and is therefore preferred in hydrogen separation applications.

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the invention. Accordingly, variousmodifications, adaptations, and alternatives may occur to one skilled inthe art without departing from the spirit and scope of the presentinvention.

1-22. (canceled)
 23. A method of forming a membrane in a plurality ofthrough-channels disposed in a substrate, the method comprising thesteps of: a. providing the substrate to a chamber, the substrate havinga first end, a second end, and a plurality of through-channels extendingthrough the substrate from the first end to the second end, wherein thechamber has an inlet section, a midsection capable of supporting thesubstrate, and an outlet section, wherein the substrate is disposed inthe midsection such that the first end is adjacent to and in fluidcommunication with the inlet section and the second end is adjacent toand in fluid communication with the outlet section, wherein the chamberis oriented vertically such that the inlet section is located at thebottom of the chamber below the midsection and the outlet section islocated at the top of the chamber above the midsection; b. providing aliquid precursor to the inlet section, wherein the liquid precursorcomprises membrane-forming materials; c. providing a pressuredifferential between the inlet section and the outlet section, whereinthe pressure differential causes the liquid precursor to flow uniformlythrough the plurality of through-channels; and d. forming the membraneon a surface of the plurality of channels.
 24. The method according toclaim 23, wherein the liquid precursor comprises a plurality of solidparticles, and wherein the step of forming the membrane on a surface ofthe plurality of channels comprises: a) transporting the plurality ofsolid particles in the precursor liquid to the surface of at least aportion of the plurality of through-channels; and b) depositing theplurality of solid particles on the surface to form the membrane. 25.The method according to claim 23, wherein the step of forming themembrane on a surface of the plurality of through-channels comprises: a.transporting the liquid precursor to the surface of at least a portionof the plurality of through-channels; and b. reacting themembrane-forming materials with the surface or each other to form themembrane.
 26. The method according to claim 23, wherein the liquidprecursor flows through the plurality of through-channels at apredetermined linear velocity.
 27. The method according to claim 26,wherein the predetermined linear velocity is in a range from about 0.01cm/s to about 200 cm/s.
 28. The method according to claim 27, whereinthe linear flow velocity is in a range from about 1 cm/s up to about 200cm/s.
 29. The method according to claim 23, wherein the step ofproviding the pressure differential between the inlet section and theoutlet section comprises providing a pressure differential between theinlet section and the outlet section in a range from about 1,000 Pa upto about 1,000,000 Pa.
 30. The method according to claim 23, whereineach of the through-channels has a diameter of less than about 3 mm. 31.(canceled)